FIG. 1A is a schematic circuit diagram illustrating a conventional inverter. The conventional inverter is connected with a load such as a three phase DC motor (not shown). As shown in FIG. 1A, the conventional inverter 100 comprises a magnetic contactor 110, a pre-charge resistor Rp, a main rectifying circuit 130, a bulk capacitor Cb, a control circuit 140 and a driving circuit 150.
The magnetic contactor 110 comprises a normally-off switch set with three switches b1, b2 and b3. Before the control signal Sc applied to a control terminal of the magnetic contactor 110 is activated, the switches b1, b2 and b3 of the normally-off switch set are in an open state. After the control signal Sc applied to the control terminal of the magnetic contactor 110 is activated, the switches b1, b2 and b3 of the normally-off switch set are in a close state.
Please refer to FIG. 1A again. A three phase AC power source (R, S, T) is connected with three AC input terminals of the main rectifying circuit 130. The pre-charge resistor Rp is connected between a DC output terminal of the main rectifying circuit 130 and a node o. Moreover, the switches b1, b2 and b3 of the normally-off switch set of the magnetic contactor 110 are connected with the pre-charge resistor Rp in parallel.
Moreover, the bulk capacitor Cb, the control circuit 140 and the driving circuit 150 are connected between the node o and a ground terminal Gnd. The control circuit 140 issues a control signal Sc to the magnetic contactor 110. Moreover, the control circuit 140 generates plural driving signals Sd1˜Sd6 to the driving circuit 150. According to the driving signals Sd1˜Sd6, the output signals Va, Vb and Vc from plural output terminals of the driving circuit 150 are adjusted to control the three phase DC motor.
Generally, a pre-charge path is defined by the main rectifying circuit 130 and the pre-charge resistor Rp collaboratively, and a main charge path is defined by the main rectifying circuit 130 and the magnetic contactor 110 collaboratively.
The operating principles of the conventional inverter 100 will be described in more details as follows.
When the three phase AC power source (R, S, T) is just connected with the magnetic contactor 110, the control circuit 140 is still disabled. Consequently, the control signal Sc is not outputted. At the same time, the electric energy from the three phase AC power source (R, S, T) is inputted into the three AC input terminals of the main rectifying circuit 130. Consequently, a DC voltage is outputted from a DC output terminal of the main rectifying circuit 130 to charge the bulk capacitor Cb through the pre-charge resistor Rp. Before the voltage of the bulk capacitor Cb reaches a steady state voltage Vdc, the control circuit 140 is disabled.
After the bulk capacitor Cb is charged to the steady state voltage Vdc, the control circuit 140 is enabled to output the control signal Sc. Meanwhile, the switches b1, b2 and b3 of the normally-off switch set of the magnetic contactor 110 are switched to the close state. Under this circumstance, the steady state voltage Vdc from the DC output terminal of the main rectifying circuit 130 is transmitted through the switches b1, b2 and b3 to charge the bulk capacitor Cb without being transmitted through the pre-charge resistor Rp.
When the control circuit 140 is enabled, the control circuit 140 generates the plural driving signals Sd1˜Sd6 to the driving circuit 150. According to the driving signals Sd1˜Sd6, the output signals Va, Vb and Vc from plural output terminals of the driving circuit 150 are adjusted.
FIG. 1B is a schematic circuit diagram illustrating the driving circuit of the conventional inverter. As shown in FIG. 1B, the driving circuit 150 comprises six transistors Q1˜Q6. The collector of the first transistor Q1 receives the steady state voltage Vdc. The base of the first transistor Q1 receives the first driving signal Sd1. The emitter of the first transistor Q1 is connected with the node x. The collector of the second transistor Q2 receives the steady state voltage Vdc. The base of the second transistor Q2 receives the second driving signal Sd2. The emitter of the second transistor Q2 is connected with the node y. The collector of the third transistor Q3 receives the steady state voltage Vdc. The base of the third transistor Q3 receives the third driving signal Sd3. The emitter of the third transistor Q3 is connected with the node z.
The collector of the fourth transistor Q4 is connected with the node x. The base of the fourth transistor Q4 receives the fourth driving signal Sd4. The emitter of the fourth transistor Q4 is connected with the ground terminal Gnd. The collector of the fifth transistor Q5 is connected with the node y. The base of the fifth transistor Q5 receives the fifth driving signal Sd5. The emitter of the fifth transistor Q5 is connected with the ground terminal Gnd. The collector of the sixth transistor Q6 is connected with the node z. The base of the sixth transistor Q6 receives the sixth driving signal Sd6. The emitter of the sixth transistor Q6 is connected with the ground terminal Gnd. Moreover, the nodes x, y and z are the output terminals of the driving circuit 150. The output signals Va, Vb and Vc are outputted from the output terminals of the driving circuit 150 to control the three phase DC motor.
Generally, the control circuit 140 generates the driving signals Sd1˜Sd6 to control the on/off states of the corresponding transistors Q1˜Q6 in order to adjust the waveforms of the output signals Va, Vb and Vc.
After the inverter 100 is fabricated, it is necessary to perform a surge immunity test. Generally, the magnetic contactor 110 can withstand a high voltage. When the main rectifying circuit 130 is enabled, a large current in response to a surge voltage of about 4 kV flows to the bulk capacitor Cb through the main rectifying circuit 130 and the magnetic contactor 110 successfully. Since the inverter 100 is not adversely affected by the surge voltage, the inverter 100 passes the surge immunity test.
However, since the volume of the magnetic contactor 110 is very large, the volume of the inverter 100 is also very large. For reducing the volume of the inverter 100, the magnetic contactor 110 is replaced by a smaller electronic component. However, the inverter 100 is possibly unable to pass the surge immunity test.